1. Field of the Invention
The invention relates to a tuneable filter, in particular for use in the visible or near infrared wavelength regions. The filter may be used, for example, in protection against laser radiation, or as a high contrast tuneable filter for display purposes. The filter may also be used for wavelength division multiplexing in telecommunications applications.
2. Discussion of Prior Art
In a conventional tuneable filter based on a Fabry Perot etalon, the cavity length may be varied in order to tune the filter to the required wavelength. This may be done by mechanically varying the cavity length, using piezoelectric drivers attached to the glass substrates at the end of the cavity. Temperature tuning techniques may also be employed, using thermo-optic effects in the material contained within the cavity. Alternatively the cavity can be tuned to a particular wavelength by varying the angle at which light is incident on the filter.
Tuneable filters may be particularly useful as a means of protecting against laser radiation, especially where the precise laser wavelength is unknown. A suitable protective device for any optical imaging system or detection system (including the human eye) must transmit in-band wavelengths so that vision or detection is not significantly obscured even when activated.